1. Field of the Invention
The present invention relates to precursor compositions that are useful for the deposition of electronic features such as resistors and capacitors. The precursor compositions can have a low conversion temperature to enable low-temperature treatment of the compositions to form passive electronic features on a variety of substrates.
2. Description of Related Art
A variety of materials are used to create electronic circuitry on a substrate. Examples include metals and other conductive materials for electrical conductors, dielectric materials for insulation and capacitive elements, resistive materials for resistors and ferroelectric materials for capacitive elements.
For example, dielectric materials have a wide variety of applications in electronic circuits. They are used to provide electrical insulation as well as to facilitate the temporary storage of electrical charge. The dielectric constant, dielectric loss factor, and dielectric strength determine the suitability for a specific application. Variations in dielectric properties with frequency, temperature and a range of environmental conditions, such as humidity, also play a big role in determining the usefulness of any particular material composition.
Traditionally, a thick film paste is deposited by screen-printing and subsequently fired (heated) to form a dielectric material. The pastes typically consist of ceramic particles and a small amount of particulate glass as a sintering aid. Annealing above 850° C. for extended times is needed to induce crystallization and densification of the material.
Historically, glasses have been widely used in the inorganic thick film industry to lower densification temperatures. Mixing glasses and ceramics has many technological advantages. In “low-k” dielectric applications where the dielectric constant (k) is about 10 or lower, a significant amount of glass is mixed with one or more refractory oxides. During firing of these materials the glass melts, resulting in a hermetic structure. The melting temperature of these glass additives is typically greater than 600° C. In some cases, low temperature sealing glasses or overglazes are used with melting temperatures in the range of 300° C. to 400° C. When such low-melting point glasses are used, entrapment of organic vehicle inside the thick film becomes a real concern. Glass additives are also used in small amounts in “high-k” dielectric applications, those having a dielectric constant of 10 or higher. During firing, the glass component melts and the liquid glass phase provides capillary pressure and a diffusion path for the ceramic phase. The amount of glass that is used in these compositions must be balanced between maximizing the effect of the glass as a sintering aid and minimizing low-k glass contamination in the processed film to achieve a high dielectric constant.
Polymer thick film technology is also used extensively and allows the formation of resistors and dielectric layers on organic and other low temperature substrates. Polymer thick-film (PTF) technology has been traditionally used in lower performance, lower cost hybrid circuits on organic substrates. This technology is based on manufacturing by screen-printing pastes consisting of polymers with conductor, resistor and/or dielectric materials and solvents on the substrate. Polymer thick films offer great versatility in the sense that materials with very different electrical characteristics and excellent adhesion to organic substrates can be obtained through simple modifications of the composition. As a result, they can be tailored to suit many applications, such as low temperature processing and flexible substrates.
Typical dielectric constants for polymers range from about 2.5 to 6, and the polymers have a high loss compared to inorganic materials. For example, polymide has a dielectric constant of 3.5 and a loss tangent of 0.01. Higher-k polymer thick films can be obtained by adding a high-k ceramic filler to the polymer. However, mixing rules for multi-component dielectric layers dictate the intrinsic limitations of this approach. Hence, there is very limited use of PTF for capacitors, particularly for stable, high performance components.
Traditional hybrid technologies also have serious limitations for today's demanding resistor applications. Screen-printed thick-film resistors for example have wide-ranging values, but their short current paths and the inherent limitations of the screen-printing resolution severely compromise their performance characteristics. Laser trimming is often required to fine-tune the values within the desired range. Thin-film resistors, while capable of high precision, are expensive to design and manufacture and have their own limitations in terms of obtainable resistance values.
Most resistors for integrated electronic applications are required to be ohmic, to have small deviations from their predetermined resistive value (tolerance), and to have small temperature coefficients of resistance (TCR). TCR is an expression of change in resistance due to change in temperature and it is expressed in parts per million per degree Celsius (ppm/° C.). The TCR of given conductive and semiconductive materials can be either positive (increasing resistance with increase in temperature) or negative (decreasing resistance with increasing temperature).
The major demand for resistors in electronic applications lies in the resistance range from 103 to 108Ω. This is a serious challenge, as pure materials with suitable and reliable electrical behavior typically have resistivities below about 10−6 Ωm. Unfortunately, there are no pure single-phase materials that provide optimum properties for ohmic resistors. The key to producing a resistor with a specific resistivity and low TCR lies in tailoring composition and microstructure of the final product and two general strategies have been successful. First, the conductivity can be lowered by diluting a conductive material with an insulative phase. Also, very thin and/or elongated conductive paths can be formed and packaged for stability and reproducibility. Thin-film resistor technology is often based on the latter approach, while thick-film materials are often mixtures of a conductive and an insulative phase.
Materials systems have been developed in the prior art for thin and thick film technologies. Thick film technology has seen the development of both polymeric type resistors as well as cermets, while thin films rely mainly on vapor deposition and lithography of metal alloys. Polymeric resistors generally consist of a polymer matrix with a conductive particulate phase such as particulates of carbon or a silver alloy. Cermet type resistors have gained popularity as they offer better performance and tolerance than polymeric resistors while offering a wide range of resistivities. A typical cermet resistor paste consists of a conductive phase and insulative matrix phase, along with other additives such as an organic carrier, binder, wetting agents and dispersants.
A major goal of the electronics industry is to produce resistors of the same quality as cermet resistors with regard to range of resistance, TCR, and stability while simplifying the process steps by lowering process temperature and time, and decreasing the process sensitivity of the material. An ideal material would retain the high quality electrical characteristics of cermets while being processable on organic substrates, including polymide substrates (e.g., KAPTON, available from E.I. duPont deNemours, and Company, Wilmington, Del.) and glass/epoxy laminates such as FR-4.
In accordance with the foregoing, the electronics industry relies on the deposition of patterns of various materials onto substrates to form circuits and passive circuit elements. The primary methods for printing of these patterns are screen-printing for features larger than 100 μm and thin film approaches for features less than 100 μm. Other subtractive processes are available for feature sizes less than 100 μm. These include photo-patternable pastes, laser trimming, and others.
U.S. Pat. No. 5,801,108 by Huang et al. discloses dielectric pastes formulated from starting materials including a dielectric powder composition, a glass composition such as a borosilicate glass that will melt at about 500° C. to 600° C. and react with the dielectric powder upon firing and partially form a crystallized phase, and a binding material such as an organic binder. The resulting dielectric paste is a multiphase dielectric paste wherein at least one phase is an alkaline earth, transition metal silicate. It is also disclosed that when the dielectric powder to crystallizable glass weight ratio is approximately 60:40, the resulting mixture will densify at approximately 850° C.
Precursor derived printable electronic compositions are described by R. W. Vest (Metallo-organic materials for improved thick film reliability, Nov. 1, 1980, Final Report, Contract #N00163-79-C-0352, National Avionic Center). Vest described only compositions that contained precursors and a solvent.
U.S. Pat. Nos. 6,036,889 and 5,882,722 by Kydd disclose conductor precursor compositions that contain particles, a metal organic decomposition (MOD) precursor and a vehicle and provide pure conductors at low temperatures on organic substrates. However, materials to form dielectrics and resistors are not disclosed. Also, formulations for fine mesh screen printing are not disclosed. Specific formulation and processing details about how to lower the decomposition temperature to below 300° C. are not disclosed.
U.S. Pat. No. 6,197,366 by Takamatsu discloses methods using inorganometallic compounds to obtain formulations that convert to dense sold metals at low temperatures.
Polymer thick film materials containing particles in a polymerizable organic vehicle have also been disclosed in the prior art. These compositions are processable at low temperatures, such as less than 200° C., allowing deposition onto organic substrates. However, these compositions are not designed for fine feature sizes such as those have a resolution of less than 200 μm. Polymer thick film also has limited performance and suffers from poor stability in changing environments. Attempts have been made to produce metal-containing compositions at low temperatures by using a composition including a polymer and a precursor to a metal. See, for example, U.S. Pat. No. 6,019,926, by Southward et al.
U.S. Pat. Nos. 5,846,615 and 5,894,038, both by Sharma et al., disclose precursors to Au and Pd that have low reaction temperatures thereby conceptually enabling processing at low temperatures to form metals.
U.S. Pat. No. 5,332,646 by Wright et al. discloses a method of making colloidal palladium and/or platinum metal dispersions by reducing a palladium and/or platinum metal of a metallo-organic palladium and/or platinum metal salt which lacks halide functionality.
There exists a need for precursor compositions, particularly high viscosity pastes, for use in electronics, displays, and other applications. Further, there is a need for precursor compositions that provide low processing temperatures to allow deposition onto organic substrates while still providing features with good electrical characteristics. Furthermore, there exists a need for a precursor composition for passive electronic features that offers enhanced resolution control.